1. Field of the Invention
The present invention relates to a catalyst, a membrane electrode assembly, a fuel cell, and a process for preparation of the catalyst.
2. Background Art
A fuel cell can convert chemical energy directly into electrical energy, and is an eco-friendly device of power generation. It, therefore, has recently attracted the attention of people. In fact, for example, a direct methanol fuel cell (hereinafter, often referred to as “DMFC”) and a polymer electrolyte fuel cell (hereinafter, often referred to as “PEFC”) have such high theoretical conversion efficiencies as 97% and 83%, respectively.
The DMFC does not need to be equipped with a reformer since liquid fuel is directly fed, and suitably works at a low temperature. Accordingly, it is particularly expected to adopt the DMFC as an alternative power supply replacing a secondary battery for cellular phones.
Generally in the DMFC, platinum is mainly employed as a methanol oxidizing catalyst at present. However, it is known that, if platinum is used as the catalyst, carbon monoxide is often generated as an intermediate product to poison the surface of platinum and, as a result, to considerably lower the catalytic activity.
In order to avoid the poisoning, many means have been studied. For example, it is proposed to replace the platinum with PtRu alloy. It is presumed that, when the PtRu alloy is used as the catalyst, oxygen species adsorbed on the Ru surface react with carbon monoxide adsorbed on the Pt surface, so as to prevent the carbon monoxide from poisoning and consequently to avoid the deterioration of catalytic activity. However, it is a problem that this means consumes a great deal of expensive noble metals. It is, therefore, very important to develop a new catalyst having high activity and less consuming noble metals.
Meanwhile, it has been also studied for years to incorporate other metals into the platinum or the PtRu alloy for further improving the catalytic activity thereof, and various reports have been submitted. For example, it is reported that alloys of platinum with base metals such as Tin and molybdenum are effective in preventing carbon monoxide from poisoning. However, this also has a problem that the additive metals are often dissolved under an acidic condition. U.S. Pat. No. 3,506,494, which was submitted in 1966, also reports the effects of ten metal additives such as tungsten, tantalum and niobium.
The catalytic reaction proceeds on the surface of nanosize catalyst particles, and hence a few atomic layers positioned at the catalyst surface give a large effect on the catalytic efficiency. Even if consisting of the same components, the catalysts may have very different surface conditions according to their synthesizing processes. For example, JP-A 2005-259557 (KOKAI) describes an anode catalyst-preparation process in which metals of 4- to 6-groups in the periodic table are incorporated to the PtRu alloy by the impregnation method. In JP-A 2005-259557 (KOKAI), it is reported that the methanol activity of the resultant catalyst greatly depends upon the order of impregnation. However, with respect to the mixing ratio among Pt, Ru and the metals of 4- to 6-groups, JP-A 2005-259557 (KOKAI) discloses only one ratio, namely, Pt:Ru:additive metal=317.7:82.3:100 by weight.
It is now still being studied to control the process for synthesizing catalyst and thereby to obtain catalyst particles having such a novel nanostructure that the resultant catalyst has higher activity than the PtRu alloy. For synthesizing the catalyst, a solution method such as the impregnation method has been hitherto generally used. However, when the catalyst is synthesized from elements hard to be reduced or alloyed, it is very difficult in the solution method to control the structure and/or surface condition of the catalyst.
On the other hand, processes of sputtering and vapor-deposition are advantageous for synthesizing the catalyst from the viewpoint of controlling the materials. However, it is not fully studied how those processes are affected or influenced by the processing conditions such as vapor elements, vapor composition, materials of the substrate, and temperature of the substrate. Since the catalyst particles are normally in the form of nanoparticles, the surface electronic state and nanostructure thereof are apt to depend greatly upon what and how much additive element is incorporated. Therefore, in order to obtain catalyst particles having high activity and excellent stability, it is thought to be necessary that favorable additive elements suitably combined in optimal amounts be incorporated into the catalyst particles.
U.S. Pat. No. 6,171,721 discloses a four-element type catalyst synthesized by sputtering. This patent describes many examples of elements usable as the additives, but is silent about the compositions or ratios of those elements. JP-A 2007-194217 (KOKAI) describes Ge in relation to a fuel cell cathode catalyst, but Ge is not indispensable and a notable effect given by Ge is not suggested. JP-A 2007-87617 (KOKAI) discloses that at least one element selected from the group consisting of Cu, Re and Ge can be used as an additive incorporated into the PtRu catalyst. However, also in this publication, Ge is not indispensable and an effect of Ge is not suggested.